1. Field of the Invention
This invention relates generally to heat transfer mechanisms, and more particularly to a heat transfer mechanism for removing the heat generated in an encapsulated circuit package assembly.
2. Prior Art
The efficient extraction of heat from integrated circuit chip packages has presented a very significant limitation on the design capability of integrated circuits. Without an efficient heat transfer mechanism, the speed and power capabilities of the circuitry of an integrated circuit package are severely limited. Early semiconductor devices solved the problem by making one of the electrodes of the device both a thermal and electrical direct contact to the external world and thereby connecting the electrode to an efficient heat transfer device, such as a studded heat sink. This was especially convenient when the electrode could be maintained at ground potential, for example. Typical discrete semiconductor devices of these configurations are found in U.S. Pat. Nos. 3,719,862 and 3,836,825. This approach was also used in some of the early approaches to the removal of heat from integrated circuit packages. For example, in an article entitled "Conduction Cooled Heat Plate for Modular Circuit Package" in Volume 13, No. 2, of the July 1970 issue of the IBM Technical Disclosure Bulletin there is disclosed a cooling technique using a conduction cooled isothermal heat plate which is metallurgically connected via an appropriate slug to the various circuit chips in an integrated circuit package. With this approach as with the approach with the prior art discrete devices, the chip then became fixed at the potential of the heat plate, which quite often was at ground potential.
In situations where the stud is connected directly to the chip, various "Z" stresses occur during the natural heat cycle the devices experience. These repeated stresses cause fatigue in the various connections within the devices, which can lead to failures of the devices.
However, there are many instances when the integrated circuit chip cannot be maintained at ground potential or at the potential of a heat sink, which presents a significant problem, since most good heat transfer mechanisms are also good electrical conductors. Many of the present integrated circuit package designs find the integrated circuit with discrete bond points to a suitable substrate enclosed by a cap in a sealed atmosphere of an inert gas. In these packages, the only thermal paths are the convection through the inert gas to the encapsulating cap and conduction through the discrete bond points to the substrate. These often experience very significant thermal resistances, which severely limit the power dissipation of the package. These restrictions are much more serious for devices that are flip-chip joined to the substrate through a series of bumps or solder balls on the active side of the chips. Here the conduction areas provided by the bumps are quite small and usually inadequate for the higher power devices. While it is possible to enhance the heat transfer from a flip chip such as by a metallurgical joining of the backside to the cap, as discussed previously, this introduces a set of stresses to the device and a serious reliability problem. In addition, it defeats the electrical isolation at the interface. Furthermore, as device costs increase, it is desirable to be able to repair defective devices. This is difficult to do when there is a metallurgical joining of both sides of the device to the package substrate and housing.
The problem can be even more severe with multichip modules, especially if they are mounted on a silicon substrate. It is extremely difficult to provide a viable metallurgical bond between a large silicon substrate and a heat sink without cracking the substrate and as a practical matter may be economically impossible.